1. Field of the Invention
The present invention relates to a heating unit using induction heating, and more particularly, to a fixing unit which fixes toner as a visualizing agent melt at predetermined temperature usable for an electrophotographic copying machine or printer.
2. Description of the Related Art
A fixing unit incorporated in a copying machine using an electrophotographic process fixes melted toner on a fixing material by heating and pressing. As a method of heating toner usable for the fixing unit, a method using the heat radiated from a filament lamp and a flash heating method using a flash lamp as a heat source are known well.
However, the fixing unit using a filament lamp heats adopts a roller body surrounding a lamp by the light and infrared ray generated by a filament lamp. Therefore, the heat transformation efficiency is 60 to 70%, considering the loss when the light is converted into heat and the efficiency when the air warmed in the roller transmits heat to the roller. Thus, it is known that the warming up takes a long time.
In the above background, the Jpn. Pat. Appln. KOKAI Publication No. 9-258586 and No. 8-76620 proposed a fixing unit using an induction heating unit as a heat source.
The Jpn. Pat. Appln. KOKAI Publication No. 9-258586 disclosed a fixing unit which heats a roller by generating an induced current (eddy current) in the roller by flowing a current in an induction coil made by winding a coil around a core provided along a rotation axis of a metallic roller.
The Jpn. Pat. Appln. KOKAI Publication No. 8-76620 disclosed a fixing unit, which is provided with a conductive film containing a magnetic field generation means, and a pressure roller coming in close contact with the conductive film, and fixes toner to a recording medium fed between the induction film and the pressure roller, by heating the conductive film.
There is a problem peculiar to a fixing unit used in a copying machine, since the size of a fixing paper (the paper pass width) is not even, the temperature at a part of the metallic roller or film becomes uneven.
The Jpn. Pat. Appln. KOKAI Publication No. 2000-206813 disclosed an example of controlling the power supplied to coils, to prevent the uneven temperature in the paper pass width, by providing a plurality of coils to meet the paper pass width, along the axial direction of a fixing roller. The fixing unit disclosed by this publication detects the fixing roller temperature at a plurality of detection points, and controls the power supplied to each coil, based on the temperatures detected at each detection point.
In the coil driving method disclosed by the Publication No. 2000-206813, the power supplied to a plurality of coils is changed at the same time. Thus, a frequency difference occurs in the power flowing in each coil, and an interference noise (whine) occurs. Further, it is necessary to provide a device for each coil to detect the power supplied to each coil.
The Jpn. Pat. Appln. KOKAI Publication No. 2001-312178 disclosed an example to supply power independently to each of multiple coils.
In the driving method disclosed by the Jpn. Pat. Appln. KOKAI Publication No. 2001-312178, there is a problem that a device which detects the power supplied to each coil must be provided independently for each coil.
Namely, in the coil driving method disclosed by the above-mentioned patent publications, it is necessary to control the power of the whole fixing unit while controlling the power at each coil, and it becomes necessary to provide a circuit to control the power at each coil, in addition to the power supply of the unit.
In the example where the coils are simultaneously driven, the power to generate in each coil is changed depending on the temperature difference in the longitudinally direction of the roller. In this case, the frequency of the inverter circuit is changed by the output. This means that two or more coils are driven with different frequencies, causing an interference noise. Particularly, when the frequency difference is larger, the interference noise becomes louder.
Either patent publication describes the method of detecting the temperature difference in the temperature detection means provided at the positions relative to each coil, and distributing the power to each coil based on the detected temperature differences. However, when changing the power distribution, the voltage fluctuation and temperature ripple are different.
Therefore, when simply changing the coil to which power is supplied, a problem may arise in the surrounding illumination, that is, a flicker may occur in a fluorescent lamp. Of course, a larger temperature ripple increases unevenness in the fixing performance.